What gave Vera confidence to weave her story about the tragedy in "The Open Window"?

If the tragedy Vera intends to tell Framton Nuttel had actually happened, it would certainly have been known about and talked of all over the county. She has to establish that Framton is a complete stranger to the area; otherwise, he might wonder why he never heard anything about the incident. If Vera's aunt had been driven insane by the gruesome tragedy, that would also be the talk of the families for years. Vera first...

If the tragedy Vera intends to tell Framton Nuttel had actually happened, it would certainly have been known about and talked of all over the county. She has to establish that Framton is a complete stranger to the area; otherwise, he might wonder why he never heard anything about the incident. If Vera's aunt had been driven insane by the gruesome tragedy, that would also be the talk of the families for years. Vera first has to establish that Framton knows nothing of any consequence. She does this by asking him two leading questions:

"Do you know many of the people round here?"

"Then you know practically nothing about my aunt?"

Framton's answers to these two questions give Vera the assurance she needs to go ahead with her story. His sister stayed at the rectory some four years ago, and the tragedy Vera describes supposedly occurred about three years ago. Framton's sister is presumably not very well informed herself. She would not have given her brother a letter of introduction to the Sappletons if she had heard Mrs. Sappleton was crazy. Framton's sister shows her own ignorance of the people in the area when she tells her brother:

"I shall just give you letters of introduction to all the people I know there. Some of them, as far as I can remember, were quite nice."

Saki invents the question-and-answer dialogue between Vera and her visitor in order to inform the reader that Framton is the perfect victim for Vera's prank.